Method and apparatus for investigating stand-off in a borehole

ABSTRACT

An acoustic method and apparatus for investigating an earth formation penetrated by a borehole are described. Acoustic transducers are mounted on a tool to accurately determine the distance between a segment of the tool and the wall of the borehole. The acoustic transducers are positioned in such manner that the stand-off distance between individual resistivity measuring electrodes in an array, which is also mounted on the tool segment, and the borehole wall can be measured. The stand-off measurement is recorded and may be used to correct the electrode resistivity measurements. In one embodiment a calibration of acoustic transducers as a function of depth is obtained by employing acoustic calibrating transducers to compensate for borehole environment effects on the performance of the acoustic transducers as well as determine the acoustic velocity of the borehole fluid, such as mud. Several embodiments are described.

FIELD OF THE INVENTION

This invention relates to acoustic investigations of an earth formationpenetrated by a borehole generally and more specifically to an acousticmethod and apparatus for enhancing earth formation investigations whichare affected by the occurrence of a stand-off distance between thesegment of the tool from where the investgation is made and the wall ofthe borehole.

BACKGROUND OF THE INVENTION

Acoustic pulse echo investigations of earth formations have beendescribed in the art. For example, U.S. Pat. No. 4,255,798 to R. M.Havira describes a method and apparatus for investigating a casing tocement bond by directing acoustic pulses from inside the borehole atradial segments of the casing and analyzing acoustic returns. The U.S.Pat. No. 3,833,841 to Norel et al describes a pad mounted pulse-echoacoustic transducer for analyzing the casing cement interface. Specialintermediate layers are employed to match acoustic impedances.

In the investigation of an earth formation, tools employing varioussources of energy may be employed. In some tools the spacing between thesegment of the tool on which the investigating energy source is locatedand the borehole wall affects the investigation and techniques areemployed to bring this segment of the tool in close proximity with thewall of the borehole. This may involve use of one or several padspressed against the borehole wall or by pressing the segment of the toolto one side of the borehole wall with tool mounted bow springs to assurethat the stand-off sensitive energy source or transducer is in closeproximity to the borehole wall. Notwithstanding use of such techniques,the tool segment may be forced to stand away from the borehole wall bythe presence of a mudcake. This stand-off may, therefore, undesirablyaffect the accuracy or interpretation of the tool's investigation of theearth formation. In some instances, cavities in the borehole wall mayappear in front of the tool segment and it is desirable to be able torecognize or at least distinguish such cavities from other formationcharacteristics as a stand-off type measurement.

Techniques have been proposed to measure mudcake thickness. For example,according to one technique, a caliper is employed to measure theborehole diameter and reductions from the original drilling diameter areinterpreted as an indication of the thickness of the mudcake. Thecaliper's use for a mudcake thickness measurement, however, requiresextensive depth shifting to relate the measured mudcake thickness tothat actually opposite the tool segment carrying the investigatingtransducer and as a result may not be sufficiently precise when a highspatial resolution investigation of the borehole wall is being made.This problem is particularly acute when the tool's logging motion isirregular, making precise high resolution depth shifting extremelydifficult.

An acoustic pulse echo technique to measure mud cake thickness isdescribed in the U.S. Pat. No. 3,175,639 to Liben. In the latter patentan indication of the acoustic impedance of a flushed zone behind themudcake is derived from a measurement of the change between an acousticreflection and the applied acoustic pulse. The acoustic pulse generatordescribed in Liben, however, is sensitive to temperature and pressurechanges encountered at borehole depths and since these render mudcakethickness measurements less precise, the temperature as a function ofdepth is computed or a temperature log is made and the pressure as afunction of depth is computed. In one technique described in Liben, anacoustic transducer is spaced at some distance from the surface of theborehole wall requiring that the borehole mud be traversed by theapplied acoustic pulse and reflections caused thereby along thisdistance. This tends to introduce inaccuracies in the mudcake thicknessmeasurement and impair resolution, due to factors such as attenuationand beam spreading. The acoustic transducer could be placed in closeproximity to the borehole wall as taught by Liben, but in such case themeasurement of relatively thin mudcake layers becomes difficult.

When a formation investigation is affected by the presence of standoff,it is desirable to measure the stand-off over a range of thickness andin a manner sufficient to resolve ambiguities introduced by the presenceof stand- off. The known prior art techniques for measuring mudcakethickness are, however, not sufficient to aid in resolving ambiguitiesor correcting measurements made in a high resolution formationinvestigation which is sensitive to a standoff condition from theborehole wall.

SUMMARY OF THE INVENTION

In a method and apparatus in accordance with the invention forinvestigating an earth formation penetrated by a borehole wherein asource of energy is used to measure a parameter which is sensitive to astand-off of the source from the borehole wall, an acoustic measurementof the stand-off is made to resolve an ambiguity in the parametermeasurement due to such stand-off. As described herein, the invention isparticularly effective in the investigation of the formation with anarray of small button electrodes capable of making a high resolutionresistivity investigation of a contiguous segment of the borehole wall.At least one acoustic transducer is located in close proximity to thearray to determine the presence of stand-off of the array from theborehole wall and thus enable one to resolve ambiguities in theresistivity investigation introduced by the stand-off.

With reference to one embodiment of the invention, a plurality ofacoustic transducers are used to determine the presence of stand-off byemploying an accurate pulse echo technique. The acoustic transducers maybe distributed in an array extending laterally across a segment of thetool so that the stand-off can be measured over an extended area. Thismay advantageously include corrections for borehole environmentaleffects such as pressure and temperature changes and the acousticproperty of the borehole fluid. The correction of stand-off is obtainedwith measurements made with calibrating acoustic transducers whichenable automatic compensation for borehole environmental effects andprovide local measurements of the acoustic wave velocity in the boreholefluid.

With one acoustic technique for determining stand-off in accordance withthe invention, stand-off measurements are made with an accuracysufficient to enable one to resolve ambiguities introduced in thestand-off sensitive parameter measurement. In addition, stand-off ismeasured with a spatial resolution that approaches the spatialresolution of the parameter measurements. For example, in one embodimentin which an array of electrodes provides a high resolution resistivityinvestigation of the borehole wall, a plurality of acoustic transducersare strategically located with respect to and in close proximity to thearray to enable a measurement of the stand-off in front of eachelectrode in the array. The ambiguities introduced by the stand-off maythen be resolved by either recording the stand-off measurementsalongside the resistivity measurements with appropriate depth-shiftingor by correcting the resistivity measurements with a deconvolutiontechnique.

It is, therefore, an object of the invention to provide a method andapparatus to determine the presence of stand-off from a borehole walland resolve ambiguities in stand-off sensitive parameter measurementsmade with a tool in the borehole. It is a further object of theinvention to measure such stand-off with a spatial resolution sufficientto complement high spatial resolution electrical measurements of theearth formation and with a precision commensurate with the sensitivityof the electrical measurement to a stand-off condition. It is stillfurther an object of the invention to provide an apparatus and method tomeasure stand-off and correct a stand-off sensitive parametermeasurement with such stand-off measurement.

These and other objects and advantages of the invention can beunderstood from the following description of several embodiments inaccordance with the invention described in conjunction with thedrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective and schematic representation of one apparatus inaccordance with the invention;

FIG. 2 is a schematic view of several operating conditions and responsecurves to illustrate the problem at which the invention is directed atan acoustic transducer used in accordance with the invention;

FIG. 3 is a typical timing diagram of a waveform generated by anddetected with an acoustic transducer employed in a method and apparatusin accordance with the invention;

FIG. 4 is a block diagram of an apparatus used to correct a measuredparameter for stand-off in accordance with the invention;

FIG. 5 is a planar front elevation view of the apparatus of FIG. 1;

FIG. 5A is a schematic representation of a tilted tool condition;

FIG. 6 is a horizontal section view taken along a plane intersecting alinear acoustic transducer array employed on the tool shown in FIG. 1;

FIG. 7 is a schematic section view of calibrating acoustic transducersemployed in accordance with the invention;

FIG. 8 is a timing diagram representative of waveforms generated anddetected by the transducers of FIG. 7;

FIG. 9 is a flow chart for a signal processor routine to derive andemploy stand-off in accordance with the invention;

FIG. 10 is a flow chart for a routine used in a signal processor tocorrect parameter measurements with stand-off measurements in accordancewith the invention;

FIGS. 11, 12 and 13 are respectively a side elevation view,cross-sectional view and perspective partially broken away view of analternate embodiment for an acoustic transducer linear array for use inapparatus in accordance with the invention;

FIGS. 14 and 15 are respectively a perspective view and side view inelevation of another acoustic array for use in an apparatus inaccordance with the invention; and

FIG. 16 is a perspective view of another acoustic array for use in anapparatus of this invention.

DETAILED DESCRIPTION OF THE DRAWINGS

With reference to FIGS. 1-4, a tool 20 for investigating an earthformation from a borehole 22 is shown connected by a cable 24 to surfacelocated equipment 26. The tool 20 includes a transducer tool segment 21which employs one or several sources of energy to investigate theborehole 22, a casing in the borehole, the wall of the borehole ordeeper located earth formations. The transducer segment 21 measures aparameter which is sensitive to variations in the distance of segment 21from the borehole. Hence, information of what this distance is while theinvestigation of the borehole is made is desirable to resolveambiguities in the measured parameter. The tool segment 21 is shown inthe form of a skid, though it should be understood that other boreholewall engaging arrangements such as a shoe or articulated pad can beused.

In the tool embodiment 20 of FIG. 1 the transducer segment 21 is formedwith an array 28 of electrodes 30 of a type selected to make a detailedresistivity investigation of a substantially contiguous area of theborehole wall 32. A tool for making such resistivity investigation maybe as described in a copending patent application entitled "Method andApparatus for Electrically Investigating a Borehole", U.S. Ser. No.288,554, now U.S. Pat. No. 4,468,623 filed July 20, 1981 by inventorsDavid E. Palaith, Stanley C. Gianzero and D. S. K. Chan and assigned tothe same assignee as of this invention. The surface of segment 21 has acurvature which is matched to the nominal curvature of the borehole wall32.

The size of the electrodes 30 is made as small as is practical, forexample as described in the above Palaith et al application, of theorder of about five millimeters in diameter, so that the array mayprovide high resolution information, of the order of severalmillimeters, about the resistivity or conductivity of the borehole wall32. The measure current I_(m) emitted by electrodes 30 are, therefore,sampled at a high rate and the samples are transmitted by cable 24 tosurface signal processing equipment 26. The latter generates a suitablerecord such as display log 34 on which the electrode's measure currentsare plotted as resistivity graphs 36 and as a function of depth.

The resistivity investigation with the array 28 involves the use ofsmall electrodes 30 to enable detection of relatively small resistivityanomalies such as presented by a fracture 38 or a boundary 40 betweenthin beds 42, 44. Although the tool 20 is provided with suitable bowsprings 50 to urge the array 28 of electrodes 30 towards wall 32 ofborehole 22, conditions may arise whereby a space, known as stand-off,is created between electrodes 30 and wall 32 of the borehole 22. Thestand-off causes the resolution of the current measurements todegenerate and logs derived from these measurements become difficult tointerpret. Such stand-off may arise, for example, as a result of thepresence of a cavity in front of the tool segment 21 or a mudcake or bya lift off or a tilt condition of the tool 20. As a result of thestand-off, the electrode current measurements lose resolution leading toambiguous interpretations. When the stand-off is detected with the sameorder of resolution as is present with the use of array 28 and isdetermined with sufficient accuracy commensurate with the sensitivity ofthe electrical measurement to the magnitude of the stand-off, smoothingeffects present in the log 34 of the electrical measurements can be moreprecisely interpreted.

The effect of a stand-off condition can be appreciated, for example,with reference to FIG. 2. In this figure a small button electrode 30 isshown at 52 in close proximity to the borehole wall 32 and capable ofinjecting a focused electrical current 53 into the formation in themanner as described in the aforementioned patent application to Palaithet al. When such close proximity condition exists as the tool is pulledup along the borehole wall, the output current from the electrode 30,when it crosses the boundary 40 between adjacent beds 42, 44 ofdifferent resistivities, is as illustrated with curve 54. In the eventthe electrode 30 crosses a fracture such as 38, the presence of itshigher conductivity causes a response characteristic as illustrated withcurve 56.

When, however, a stand-off condition arises such as shown at 57 in FIG.2 where a mudcake 60 has developed between the electrode 30 and theborehole wall 32, the current distribution is less focused and theresponses of the electrodes to surface anomalies such as fractures orthin bed boundaries are likely to be as shown with curves 58 and 60.These curves 58, 60 exhibit a smoothing of the electrode measurecurrents as if slower formation changes occur than is actually the case.Since the high resolution of the array, of the order of 2.5 mm, isintended to provide a detailed "picture" of the wall 32, the presence ofmudcake 60 or a tilt condition seriously affects the array measurements.

Accordingly, a stand-off investigating acoustic transducer is employedin close proximity to the microresistivity measuring array 28 so as toobtain a measurement of the stand-off with sufficient spatial resolutionthat the stand-off opposite each of the individual electrodes 30 in thearray 28 can be derived. In the embodiment as illustrated in FIG. 1, anumber of stand-off investigating transducers 64, are provided, eachoriented to direct acoustic pulses towards the borehole wall and senseacoustic reflections caused at interfaces of media having contrastingacoustic impedances such as the wall 32 behind the mudcake 60. Withacoustic transducers 64 accurate measurements of stand-off can be madewith a spatial resolution generally of the same order as the spatialresolution capability of the electrodes 30 in the array 28 and with anaccuracy commensurate with the sensitivity of the electrode currentmeasurements to stand-off conditions. The stand-off measurement SO asderived with signal processor 26 is recorded as log SO as a function ofdepth on log 34.

As illustrated in the view of FIG. 5, stand-off detecting transducers 64are distributed at precisely known distances from and in close proximityto array 28. Hence, the measurements of the magnitude of the stand-offwith these transducers can be accurately depth shifted and interpolatedto derive the amount of stand-off opposite each of the electrodes 30 inarray 28.

For example, if the tool 20 exhibits a significant tilt as illustratedin exaggerated fashion in FIG. 5A, the resulting different stand-off asmeasured with transducers 64.1-10 and 64.11, 64.12 can be determined. Anappropriate geometrical process can be implemented to derive the degreeof stand-off (SO) opposite the respective electrodes 30 in array 28.

Although a single acoustic transducer 64 could be employed to determinestand-off, when an electrical array 28 is used there are a number offactors making it preferable that more than a single acoustic transducer64 be employed. For example, the borehole surface opposite the electrodearray 28 is likely to have small irregularities leading to differentresistivity responses by different electrodes 30 in the array 28. Or,the borehole cross-sectional shape is such as to cause unpredictabletool orientations relative to the borehole wall 32. Since the effect ofthese factors on the resistivity measurements are unlikely to beresolved with the stand-off measurement by a single acoustic transducer64, preferably several acoustic transducers 64 are used so as to be ableto measure the stand-off opposite each of the electrodes 30 in thearray.

In the tool embodiments shown in FIGS. 1 and 5, the acoustic transducers64 are laterally spread in a linear array 66 vertically below theelectrode array 28 and with a pair of spaced transducers 64.11 and 64.12above array 28. The use of linear array 66 enables the acousticdetection of a small cave which, when bridged by the array so as to beopposite several but not all of the electrodes 30, would causeambiguities in the electrode measure currents. These ambiguities areresolved by the measurements made with the linear acoustic transducerarray 66.

The spatial resolution of the acoustic array is, therefore, selected soas to be able to distinguish the presence of a small cave opposite theelectrical array from the smoothed electrical responses obtained fromthe electrodes 30 opposite such cave. Thus, the spatial resoltuion ofthe acoustic array 66 preferably is in the range from that of theelectrical array 30 to about five times that. This enables therecognition of a cave bridged by array 28 and larger than thecrossectional area of an electrode 30. Since the stand-off does notnormally vary significantly over short distances, the acoustic array 66may be formed with spaced-apart acoustic transducers 64 as shown inFIGS. 1 and 5. The number of acoustic transducers 64 in the array 66being then selected to obtain a spatial resolution of the order that isgenerally commensurate with the spatial resolution of electrical array28.

The acoustic transducers 64 may be made in the manner as moreparticularly described in the aforementioned U.S. patent to Havira. Thetransducers 64 as shown in FIG. 2 are each formed with a piezoelectricelement 65 which is located between an acoustic backing material 63 andan acoustic delay line 70 whose end interface surface 72 terminatessubstantially at the same surface as for electrodes 30 on tool 20.

Different types of transducers 64 may be used. The transducers shown inFIGS. 1-7 are of a cylindrical design with a diameter D of about 6 mm.The excitation pulses for transducers cause transmission of acousticwaves at a frequency of the order of about one MHz. At this operatingfrequency the transducers operate with a near field region out to adistance of D² /λ so that the field pattern remains essentiallycollimated over the stand-off distances of interest, i.e. generally lessthan about 20 mm though a larger stand-off can be accommodated. Thespatial resolution of such transducer is on the order of 5 to 10 mm.

The selection of these transducer dimensions and operating frequency mayvary. For example, the diameter D, see FIG. 5, may be increased toobtain a longer collimated near field region with less sensitivity torough wall surfaces. A larger diameter, however, normally yields lessspatial resolution over the stand-off distances of interest.

The transducer frequency may be increased to obtain a better rangeresolution and a longer near field region. However, such higherfrequency increases sensitivity to surface roughness and is subject tohigher attenuation.

The acoustic delay line 70 may be increased in length, l, to increasethe maximum observable stand-off and decrease source ring-down effectsif there are any. However, too long an acoustic delay line 70 decreasesthe firing rate and increases the decay time of reverberations so as tolikely result in poorer spatial resolution. A delay line 70 for use atan operating frequency of one MHz and with a transducer diameter ofabout 6 mm may have a length of about 10 mm. The length of the delayline 70 is selected sufficient so as to allow sufficient "ring-down"time and thus prevent the incidence of an acoustic return on transducer64 before its energizing pulse has terminated. The length is representedby the time T1 that it takes for an acoustic pulse to travel to endinterface surface 72. Since the interval T2 used by an acoustic pulse totravel from the surface 72 to borehole wall 32 is to be measured as anindication of the stand-off, SO, the transducer 64 is also employed tosense acoustic returns. The stand-off interval T2 is derived from thedetection of the acoustic reflection of the acoustic pulse from boreholewall 32. A stand-off indication may be obtained by measuring theround-trip travel time for an acoustic pulse to travel to wall 32 andsubtracting from that the round-trip travel time to end interfacesurface 72.

The high resolution of the electrode array 28 requires that thestand-off, SO, be measured with a corresponding spatial resolution andwith an accuracy commensurate with the sensitivity of the arrayresistivity measurements to stand-off. This imposes a high degree ofaccuracy on the stand-off measurement. Generally, when a stand-offsensitive electrical measurement is made with an array 28 as describedin the aforementioned Palaith et al application, stand-off should bemeasured so as to be able to distinguish thicknesses differing from eachother by about five millimeters. There are a number of factors, however,which tend to degrade the accuracy of an acoustic measurement of thestand-off. For example, changes in temperature and pressure encounteredby the tool 20 in the borehole affect the propagation time through delayline 70 and thus the accuracy of transducers 64 and the mudcake acousticvelocity may vary as a function of borehole depth.

Tool 20 employs calibration acoustic transducers 74, 76 with whichrespectively the effect of the borehole environment on the stand-offinvestigating transducers 64 and the acoustic velocity of the boreholefluid can be measured as a function of borehole depth. Transducer 74enables measurement of the travel time T₁ of acoustic pulses throughdelay lines 70. Acoustic transducer 76 measures the velocity of theborehole fluid, V_(m), through a slot 78 of known width and exposed toborehole fluid. The borehole fluid acoustic wave velocity V_(m)approximates that of the mudcake.

Measurement of the borehole fluid acoustic wave velocity may be done asshown with a slot 78 cut in a lower portion of the tool 20 projectingsegment 21 on which the array 28 of electrodes 30 is located.Alternatively a slot or gap 78 may be at another suitable place on tool20 but preferably at such location where the borehole fluid in the gap78 is able to pass through so that the velocity measurements are madefor borehole fluid at a borehole depth in the vicinity of where thearray 28 is employed. Gap 78 is, therefore, preferably at the side orback of the tool segment 21.

Actuation of the electrode array 28 and sampling of measure currentsI_(m) is done with a network 84 as more particularly described in theaforementioned patent application to Palaith et al. The sampled measurecurrents I_(m) on output line 86 are transmitted along cable 24 tosignal processor 26.

Energization, detection of acoustic reflections and measurement of timeintervals with acoustic transducers 64, 74 and 76 is done with the aidof a network 88 similar to that as described in the Havira U.S. Pat. No.4,225,798. The network 88 includes a signal processor 90 which actuatesthe transducers in sequence through pulsers 92 and enables time intervaldetections of reflections through a gated amplifier 94. In additionentire reflections are gated in through a return detector amplifier 96having a threshold level set by a network 98 controlled by signalprocessor 90.

With network 88 the acoustic reflections such as 100 in FIG. 3 from theend surface/mud interface 72 and the reflections such as 102 from theborehole wall 32 are detected and analyzed to generate appropriatesignals for a precise determination of stand-off. These signals includethe time intervals ΔT_(F) on line 104 for the arrival of the wallreflection 102 since the start of the acoustic pulse 106 which causedthe reflection and the magnitude E_(F) on line 108 of wall reflection102, such as its peak level or energy content.

In addition, a calibration section 110 in signal processor 90 provides atime interval signal ΔT_(CAL) on line 112 representative of the time T₁for a pulse 106 to travel through delay line 70. A signal E_(m) on line114 and representative of the magnitude of the transducer/mud interfacereflection 100 is produced. A signal representative of the boreholefluid acoustic wave velocity V_(m) is produced on line 116 in the formof a measured time interval ΔT_(mud) for an acoustic pulse such as 106to traverse a known distance through the mud, such as through the widthof slot 78.

FIGS. 5 and 6 illustrate the placement of acoustic transducers 64 withrespect to electrode array 28 with greater detail. The array'selectrodes 30 are small, of the order of 2.5 mm in radius, r, and areshown closely spaced to each other in overlapping relationship inmultiple rows spaced at regular distances L. The measure currents I_(m)from these electrodes 30 are sampled at sufficient frequency so that aresistivity pattern of the borehole wall can be obtained with aresolution measured on the order of millimeters. With such highresolution fine borehole wall details can be measured.

With such high resolution, however, the effect of stand-off tends to besignificant and the stand-off if any should be measured to resolveambiguities in the electrode measure currents I_(m). Measurement ofstand-off, however, should be done at close vertical distances fromarray 28 in order to minimize errors introduced when the stand-offmeasurements are depth shifted to determine the stand-off conditions infront of the electrodes 30. The stand-off investigating acoustictransducers 64 are, therefore, spaced at predetermined locations andclose depth-shiftable distances, d, of the order of millimeters from thearray 28.

The spacing, d, between the electrical array 6 and stand-off measuringtransducers 64 may be varied, preferably as small as possible. Whentransducers 64 are close to electrical array 28, the depth shifting ofstand-off measurements results in a more precise determination of thestand-off opposite the electrodes 30. However, too close a spacing d mayinterfere with the mechanical wiring and space needs of the electricalarray. When the spacing d is large, the stand-off measurements are moredifficult to accurately align by depth shifting with the electricalmeasurements made with electrical array 28.

Since the acoustic transducers 64 are in turn sensitive to boreholeenvironmental conditions, calibrating transducer 74 is employed todirect acoustic pulses at a fixed reflective target 130 which is ofhighly contrasting acoustic impedance and is located at the end surfaceof the same type of delay line 70 as employed with transducers 64. Thispermits a precise measurement of the acoustic travel time through delayline 70 while this is affected by local borehole conditions.

The mud calibrating transducer 76 is located in direct, through its endsurface interface 72, acoustic pulses at a remote target 132 in the formof slot wall 134 which is at a precisely known distance S from the endsurface 72.

With reference to FIGS. 7-9 apparatus and use of the calibrationtransducers are shown. When signal processor 90, see FIG. 1, commenceswith actuation of transducer 74 at a time t_(o) a significant returnreflection 100 is generated by the fixed target 130 so that at the endof an interval corresponding to the travel time through delay 70 areturn is detected at time t_(a). At another time transducer 76 isactuated to generate a pulse 106 which travels through delay 70. Suchdelay 70 is formed of a material whose acoustic impedance approximatesthat of the borehole fluid as closely as possible so that the reflectionarising at interface 72 should be very small. Notwithstanding suchmaterial selection, however, impedance mismatches are likely to occur atthe end interface surface 72 leading to a significant reflection 100. Ashort interval later the acoustic pulse transmitted through the space Sis incident upon target 132 causing detection of a reflection 140 at atime t_(b).

FIG. 9 illustrates a signal processor routine 148 for operating theacoustic transducers 64, 74 and 76 and employing the measurements madetherewith. The routine 148 includes portions which may be carried out inwhole or in part downhole by signal processor 90. Commencing at 150calibrating transducer 74 (referred to in FIG. 9 as TU_(CAL)) isactuated and the time t_(o) this is done, stored at 152. At 153 a valuefor a threshold is set. This threshold level is selected sufficientlyhigh to avoid responding to system noise, yet not too high lest theequipment is unable to detect the echo from end surface 130 or theacoustic reflection from the transducer/mud interface 131 of transducer76 at slot 78. The selection of a threshold as referred to herein mayalso be obtained by the control of the gain of a variable gain amplifier(VGA) whose input is coupled to a transducer 64 through a multiplexer asshown in the U.S. Pat. No. 4,255,798 to Havira. In such case, acomparator compares the output of the VGA to a fixed reference level anddetects an acoustic reflection when the amplifier output exceeds thisreference level.

A waiting sequence is then begun to determine at 154 whether reflection100 has arrived and when this is detected the time of arrival, t_(a),stored at 156. The difference in time between t_(a) and t_(o) isdetermined at 158 and is the time interval ΔT_(CAL) associated with thetravel of acoustic pulses through delay lines 70 of transducers 64, 74and 76.

Transducer 76 (Tu_(mud)), used for measuring the acoustic velocity ofthe borehole fluid, is energized at 160 and the time this is done isstored at 162. A time window is then selected at 164 with a timeduration commensurate with the maximum interval at which an acousticreflection from the delay line/mud interface 72 should occur. A check isthen made at 166 whether a reflection 100 has been detected and if not,a test is made at 168 whether the time window has passed. The lattertest allows for the event when acoustic impedances of the delay 70 andthe borehole fluid are so closely matched that the acoustic reflection100 is too small to detect. Hence, if the time window has timed out alower threshold, TH, is set at 170 and a return is made to step 160. Thelowering of the threshold is done preferably with a small decrement. Theprocess of energizing transducer 76 and decrementing the threshold iscontinued until an interface reflection is detected at 166. The timet_(a) at which the latter reflection is detected is stored at 172. Theamplitude A of the reflection at the delay line/mud interface 131 ismeasured at 174. Care should be taken not to reduce the threshold at 170below a level at which noise instead of an echo is detected.Accordingly, a test is made at 175 whether the threshold, TH, has beendecremented to its lowest acceptable level. Such level is a function ofsystem noise and is pre-set at some level above that. When the testindicates a lowest threshold level the next step is carried out at 176where the value for time t_(a) is set equal to ΔT_(CAL) and the echoamplitude at a reference value A_(R).

A check is then made at 178 whether a wall reflection 140, i.e., theacoustic reflection of the slot wall 132 has been detected in excess ofthe last decremented threshold value TH. If not, a waiting cycle isentered, which may be escaped, if at 179 the waiting time exceeds amaximum T_(MAX). The time t_(b) of arrival of reflection 140 is storedat 180. The mud calibration interval ΔT_(mud) may then be determined at182 as the difference between the time t_(b) and t_(o) and bysubtracting from this difference the travel time ΔT_(CAL) representativeof the acoustic pulse round-trip travel time through delay line 70. Theacoustic wave velocity of the borehole fluid may then be calculated at184 according to the relationship

    V.sub.m =2 S/ΔT.sub.mud.

Once transducers 74, 76 have been used to generate signalsrepresentative of the calibration ΔT_(CAL) of the delay lines 70 and theacoustic wave velocity of the borehole fluid V_(M), the stand-offinvestigating acoustic transducers 64 are energized commencing with thefirst transducer, Tu₁, at 190 and the time this is done is stored at192. A threshold is set at 191 with a value A+Δ which is slightly higherthan the measured amplitude A for the reflection 100 (see FIG. 8) atinterface 72 (see FIG. 7) and as measured at 174. With this thresholdthe detection of a reflection from the borehole wall 32 with a smallamount of stand-off can be made. The detected returns from firing of thefirst transducer Tu₁ are analyzed at 194 for arrival of thewall-reflection 102 (see FIG. 3) by sensing whether the reflection 102exceeds the threshold value TH as set at 191. If the wall reflection hasarrived, its time of arrival, t_(f), is stored at 196 and its energycontent E_(f) measured.

In the event no wall reflection is sensed at 194 from the firing oftransducer Tu₁, a waiting cycle is entered. If the time lapsed since theactuation of the transducer Tu₁ at 190 exceeds the delay intervalΔT_(CAL), as checked at 202, a new lower threshold level TH, less thanA+Δ as generated at 191, is set to be effective thereafter at 204. Thelower threshold enables the subsequent detection of a small boreholewall return whose peak magnitude is less than the reflection arising atthe transducer/mud interface 72. Change to a lower threshold level, TH,enhances the sensitivity in the detection of smaller returns from a moredistantly located mudcake interface. The lower threshold level, however,is not set so low as to detect noise spikes. A check is then made at 205whether an acoustic reflection has been detected which exceeded thelower threshold as set at 204. If so, a return is made to 196 to storethe time of arrival t_(a) and measure the magnitude E_(f) of the wallreflection.

When no return is detected at 205, a test is made at 206 whether theelapsed time exceeds a maximum T_(MAX). If not, a return is made to step205 to again look for a wall reflection. In the event no wall reflectionis detected and the maximum time has elapsed, a maximum value for thearrival time t_(f) is set at 208 and a return is then made to step 212.

At step 212 a check is made whether the above process needs to beexecuted for another transducer Tu. If so, a counter representative ofthe number of transducers 64 is incremented at 214 and the nexttransducer Tu₁ is then energized at 190.

After all transducers 64 (TU) have been operated and wall reflections102 have been detected the interval due to stand-off, if any is present,is determined at 218 for each transducer by determining the intervalΔT_(f) for the wall reflection 102 and subtracting therefrom theinterval for the delay line ΔT_(CAL). This is done for each stand-offinvestigating transducer 64 and in sequence and if desired the intervalmeasured with the transducers 64 can be recorded at 220 asrepresentative of the amount of stand-off.

A more precise determination of stand-off, SO, is obtained, however, bymultiplying the measured interval representative of stand-off by themeasured acoustic wave velocity V_(M) at 222 and recording this at 224.

The electrode measure currents I_(m) are sampled at 226 eithersimultaneously with the operation of the acoustic transducers or insequence. The sampled current values are recorded at 228 and a returnmade at 230 to the beginning of routine 148 at step 150.

The routine 148 is cycled through at a rapid speed. In this mannervertical motion of tool 20 can be made to have but a small effect on thevertical resolution of the stand-off investigating acoustic transducers64. The cycling speed may vary depending upon the speed of movement oftool 20, but may be of the order of several kiloherz.

With the measurement of stand-off, an improvement of the attendant lossof resolution by the electrode array 28 can be achieved using adeconvolution technique. This may be implemented as shown in FIG. 4 byapplying the stand-off, depth and and electrode current measurements toa stand-off corrector 240. The latter represents a routine for a signalprocessor with which a deconvolution of the electrode current data canbe executed. Such deconvolution may be applied to electrode currentsI_(m) for which stand-off is approximately constant. Such deconvolutionscheme may follow well known steps as described in an article entitled"On The Application of Eigenvector Expansions to NumericalDeconvolution" and published by M. P. Ekstrom and R. L. Rhoads at page319 in the Journal of Computational Physics, Vol. 14 No. 4, April 1974and an article entitled "Removal Of Intervening System Distortion ByDeconvolution" by the same authors but published in the IEEETransactions On Instrumentation And Measurement, Vol. IM-17, No. 4, page333 of the December 1968 issue.

The deconvolution technique of resolution corrector 240 may be carriedout by the steps of the corrector routine in FIG. 10. Thus, commencingat 242 the stand-off for each button electrode 30 in the array 28 ismeasured using the apparatus and steps as previously described with thestand-off investigating acoustic transducers 64 and with the informationderived with calibrating transducers 74, 76.

At 244 a system function H_(d) (x, z) associated with the measuredstand-off is derived. This may be done by storing a numerical set ofresponse characteristics for different stand-off values, such as, forexample, from zero stand-off to 15 mm of stand-off at intervals of say 5mm, though different interval values may be employed.

At 246 the two dimensional borehole wall region (x, z) is separated intoregions of approximately constant stand-off. At 248 these identifiedregions of generally constant stand-off are subjected to a deconvolutionprocess in a manner described in the foregoing articles to remove or atleast moderate the resolution loss due to stand-off and producestand-off corrected resistivity values for recording at 250.

Variations of the described embodiments may be implemented. For example,the array of acoustic transducers 64.1-10 may be mounted in a differentlinear array configuration wherein each transducer directs its acousticbeam at a common reflector for its direction at the borehole wall. Thisis illustrated in the embodiments of FIGS. 11-16.

With reference to FIGS. 11-13, a linear array 260 of acoustictransducers 64 is shown mounted in a recess 262 below electrode array28. The transducers 74 are located at an interface 264 which is oppositean acoustic reflecting surface 266. The recess 262 is filled with amaterial 268 which serves the function of acoustic delay line 70 and is,therefore, as closely matched as possible in its acoustic impedance tothat of the borehole fluid. The linear array 260 is aligned along acylindrical surface whose curvature is preferably selected commensuratewith that of the borehole wall 32 in which the tool is expected to beused. The reflector surface 266 normally bears an angle of about 45°relative to the direction of the acoustic beams from the transducers 64and the surface 269 of tool segment 21. The size of recess 262 and thusthe length, 1, of the path traveled by the acoustic pulses and acousticreturns is selected commensurate with the delay desired as previouslyexplained with reference to delay line 70.

FIGS. 14 and 15 illustrate yet another linear array 280 of acoustictransducers 64. The array 280 is formed of a pair of arrays as shown inFIG. 11 with reflecting surfaces 282, 284 in adjacent recesses 286, 288.The recesses 286, 288 are filled with acoustic impedance matching anddelay material 268. The array 280 enables a close spacing of theacoustic transducers 64 and, as shown in FIG. 15, a contiguous acousticinvestigation of the stand-off in the borehole wall area opposite theelectrical array 28.

In FIG. 16 an acoustic transducer array 290 is shown suitable for usewith an electrode array 28. The array 290 is formed with a layer 292 ofacoustic pulse generating material overlying and attached to an acousticabsorbent layer 294. Layer 292 is scribed to divide it into separatelyexcitable transducers 64, each of which generates a beam of acousticenergy through delay line layer 70'.

Determination of the presence of a tilt condition as illustrated in FIG.5A can be made as well as for a lift-off condition where a lateral sideof the tool segment 21 is lifted away from the borehole wall 32. Thetilt condition can be derived from a consistent difference over somedepth in the stand-off measured by vertically spaced and generallyaligned stand-off investigating transducers 64.1 and 64.11 for example.Similarly a lift-off condition can be detected from a consistentdifference in the stand-off measured by laterally spaced transducerssuch as 64.1 and 64.10.

Having thus described an appartus and method for determining thestand-off of a borehole tool segment from which a stand-off sensitiveparameter is measured, the advantages of the invention can beappreciated.

Description of the embodiments herein are, therefore, to be illustrativeof the invention, the scope of which is to be determined by thefollowing claims.

We claim:
 1. A method for investigating an earth formation penetrated bya borehole with a tool having a segment which is provided with alaterally extending array of small current emitting electrodes arrangedto make resistivity measurements in such spatial relationship that acontiguous lateral area of the borehole is investigated as the toolsegment, while being pressed towards the borehole wall, is operativelymoved along the borehole wall, said electrodes being sized to enable ahigh resolution resistivity investigation of the earth formation with aresolution of the order of millimeters, comprising the stepsof:generating high resolution beams of acoustic energy in the form ofpulses and directing these at segments of the borehole wall from placesthat are from at least laterally separated places that are generally invertical alignment with the array of electrodes and are pressed with thearray towards the borehole wall; detecting at said places acousticreflections originating from the borehole wall segments and caused bysaid pulses of acoustic energy; and deriving from the detected acousticreflections acoustic travel times indicative of the magnitude of toolstandoff at said places as well as lateral tool lift-off at said arrayof electrodes with an accuracy and resolution sufficient to resolve, insaid resistivity measurements, ambiguities attributable to stand-off,and vertical and lateral tool tilt.
 2. The method as claimed in claim 1wherein said generating step further includes:generating said acousticbeams with a spatial resolution generally of the same order as thespatial resolution of said resistivity investigation.
 3. The method asclaimed in claim 1 wherein said directing step furtherincludes:directing said pulses from locations vertically spaced bothabove and below the array of electrodes and extending in an arraylaterally across the tool segment and, wherein the derived acoustictravel times are also indicative of vertical tool tilt.
 4. The method asclaimed in claim 1 and further including the steps of:detecting acousticreflections of test pulses arising at an interface of a fluidcalibrating transducer with borehole fluid; determining the magnitude ofsaid latter reflections; and wherein said step of detecting acousticreflections detects acoustic reflections which exceed said determinedmagnitude.
 5. The method as claimed in claim 1 and further including thesteps of:directing first acoustic test pulses from a first calibratingtransducer at a first target on the tool having a fixed known positionrelative to the first calibrating transducer; detecting reflections ofsaid test pulses from the target; determining, from said detectedreflections, signals representative of the calibration as a function ofdepth of earth formation investigating transducer used to generate saidhigh resolution beams; directing second acoustic test pulses from afluid measuring transducer through borehole fluid at a second target onthe tool having a known distance from said fluid measuring transducer;detecting reflections from said second target; determining, from saidlatter reflections, signals representative of the acoustic velocity ofthe fluid in the borehole; and deriving from the detected acousticreflections from the borehole segments and from said first and secondtargets an indication of the magnitude of stand-off of said array thatis corrected for local borehole conditions to produce said stand-offindications with an accuracy that is sufficient to resolve ambiguitiesin said resistivity investigation attributable to stand-off.
 6. Themethod as claimed in claim 5 and further including the stepsof:measuring first time intervals between the generation of pulses andthe detection of reflections caused thereby from the wall of theborehole; measuring second time intervals between the generation of saidfirst acoustic test pulses and the reflections caused thereby from thefirst target; subtracting said second time intervals from the first timeintervals to derive third time intervals representative of thestand-off; and modifying said third time intervals with said signalsrepresentative of the acoustic velocity of the borehole fluid to derivean indication of a stand-off adjusted for bore hole conditions asmeasured at the depth where said stand-off is derived.
 7. A method forinvestigating an earth formation penetrated by a borehole with a toolemploying a plurality of similar earth formation investigating acoustictransducers for measuring acoustic parameters of the earth formation,comprising the steps of:during an investigation by the tool within theborehole directing acoustic test pulses from an acoustic transducer atan acoustic reflection target which is located so as to enablemeasurement of an acoustic characteristic of the latter transducer as afunction of borehole depth; detecting acoustic reflections from thetarget with the latter acoustic transducer and caused by said acoustictest pulses; deriving from said detected reflections measurements of anacoustic characteristic of the latter transducer as a function ofborehole depth; and applying the derived measurements so as to enable acorrection of measurements made with said earth formation investigatingtransducers for variations in the performance of said latter transducersas a function of borehole depth.
 8. The method as claimed in claim 7wherein said deriving step further includes the steps of:measuring firsttime intervals between the generation of acoustic pulses from an earthformation investigating transducer placed on the tool and the detectionof reflections caused thereby from the wall of the borehole; measuringsecond time intervals between the generation of said acoustic testpulses and the detection of reflections thereof from the target; andsubtracting from said first time intervals said second time intervals todetermine a third time interval indicative of stand-off of the tool fromthe borehole wall.
 9. The method as claimed in claim 8 wherein saidderiving step still further includes the steps of:measuring a boreholefluid parameter representative of the acoustic velocity of the boreholefluid as a function of depth and applying said measured borehole fluidparameter to said measured third time interval to adjust said stand-offmeasurement for local values of the acoustic velocity of the boreholefluid.
 10. An apparatus for investigating an earth formation penetratedby a borehole with a tool having a segment which is provided with alaterally extending array of small current emitting electrodes arrangedin such spatial relationship that a contiguous lateral area of theborehole is investigated as the tool segment, while being pressedtowards the borehole wall, is operatively moved along the borehole wall,said electrodes being sized to enable a high resolution resistivityinvestigation of the earth formation with a resolution of the order ofmillimeters, comprising:stand-off investigating acoustic transducermeans for generating and directing high resolution beams of acousticenergy in the form of pulses towards segments of the borehole wall fromplaces that are above and below the array of electrodes and at laterallyseparated places that are generally in vertical alignment with the arrayof electrodes and detecting at said places acoustic reflectionsoriginating from the borehole wall segments from acoustic pulsesrespectively generated at said places; means for measuring, at saidplaces, acoustic travel times of the acoustic pulses to the boreholewall and producing signals indicative of the magnitude of tool stand-offat said places as well as vertical and lateral tool tilt at said arrayof electrodes, with said measured acoustic travel times being measuredwith an accuracy and resolution sufficient to resolve, in saidresistivity measurements, ambiguities attributable to stand-off, andvertical and lateral tool tilt.
 11. The apparatus as claimed in claim 10said transducer means includes an array of acoustic transducerslaterally spaced on said tool segment in vertical depth-shiftableproximity to said array of electrodes.
 12. The apparatus as claimed inclaim 10 wherein selected ones of said stand-off investigating acoustictransducer are arranged in a laterally extended array having a knownproximate, vertically spaced depth-shiftable position with respect tothe array.
 13. The tool as claimed in claim 10 and furtherincluding:means for correcting said resistivity investigation as afunction of the magnitude of the stand-off at said electrodes in thearray.
 14. The apparatus as claimed in claim 10 and further including acalibrating acoustic transducer; and means responsive to detectedacoustic reflections caused by acoustic pulses from the calibratingacoustic transducer for generating calibration signals representative ofthe calibration of the stand-off investigating transducer means as afunction of depth.
 15. The apparatus as claimed in claim 14 wherein saidmeasuring means includes;means for detecting said reflection signalswhen they exceed a predetermined threshold level; and means responsiveto the calibration signals for reducing said threshold level when anacoustic reflection has not been detected within a time perioddetermined from said calibration signals.
 16. The apparatus as claimedin claim 14 wherein said measuring means includes;means for applyingsaid calibration signals to reflection signals from said stand-offinvestigating acoustic transducer means to compensate said signalsindicative of stand-off for the effect of borehole conditions on saidtransducer means.
 17. The apparatus as claimed in claim 16 wherein saidcalibrating acoustic transducer is of like construction as saidstand-off investigating acoustic transducer means and is provided with atarget to provide an acoustic calibration reflection for detection bythe calibrating acoustic transducer.
 18. The apparatus as claimed inclaim 16 wherein said means for producing signals indicative ofstand-off further includes:means for producing interval signalsrepresentative of the time intervals measured from the time the acousticpulses giving rise to the acoustic reflections were generated until theacoustic reflections are detected; and wherein said applying meansincludes means for extracting interval signals related to thecalibration signals from the interval signals related to the reflectionsignals from the stand-off investigating transducer means.
 19. Theapparatus as claimed in claim and further including a borehole fluidmeasuring acoustic transducer; andwherein said means for producingsignals indicative of stand-off includes means responsive to detectedacoustic reflections caused by acoustic pulses from the borehole fluidmeasuring acoustic transducer for producing borehole fluid calibrationsignals representative of the acoustic wave velocity of the boreholefluid as a function of depth, and means for applying said borehole fluidsignals to said signals indicative of stand-off to obtain a boreholefluid corrected indication of the magnitude of the stand-off at saidarray.
 20. The apparatus as claimed in claim 19 wherein said means forgenerating borehole fluid signals includes means for establishing aborehole fluid filled path of known width and located in the path ofacoustic pulses from the borehole fluid measuring transducer, saidlatter means including a target to cause acoustic reflections to returnto said borehole fluid measuring transducer.
 21. An apparatus forinvestigating an earth formation penetrated by a borehole with a toolemploying a plurality of similar acoustic earth formation investigationtransducers for measuring acoustic characteristics of the earthformation, comprising:a calibrating transducer similar to said earthformation investigating transducers and mounted on said tool to generateacoustic test pulses, an acoustic reflection target being so mounted asto enable the calibrating transducer to detect reflections from thetarget and enable it to measure an acoustic characteristic of thecalibrating transducer as a function of borehole depth; means responsiveto detected acoustic reflections from the target for generatingcalibration signals representative of the acoustic characteristic of thecalibration transducer as a function of borehole depth; and means forapplying the calibration signals so as to enable correction ofmeasurements made with the earth formation investigating transducers forvariations in the performance of said latter transducers as a functionof borehole depth.
 22. The apparatus as claimed in claim 21 and furtherincluding:a delay line mounted in opertive relationship with saidcalibrating transducer and terminating at an end surface opposite tosaid latter transducer, said target being mounted to said end surface toreflect said acoustic test pulses.
 23. The tool apparatus as claim inclaim 21 and further including:acoustic transducer means for calibratingthe borehole fluid and producing a borehole fluid signal indicativethereof; means responsive to the borehole fluid signals and thecalibration signals for correcting acoustic characteristics measuredwith the earth formation transducers for variations in the acousticvelocity of the borehole fluid and pressure and temperature in aborehole.
 24. A method for investigating an earth formation penetratedby a borehole with a tool employing on a segment thereof a micro arrayof electrodes for measuring earth formation resistivity at discreteportions in such spatial relationship as to enable a high resolutionresistivity investigation over a circumferentially contiguous area ofthe borehole wall and with the resistivity being measured with a highresolution of the order of millimeters and wherein the resistivitymeasurements are sensitive to the occurrence of a stand-off condition ofsaid micro electrodes from the borehole wall, comprising the stepsof:directing pulses of acoustic energy from a plurality of toollocations determined by an array of stand-off investigating acoustictransducers towards the borehole wall at circumferentially discretesegments thereof wherein the tool locations have a close depth shiftableproximity to the electrodes in the micro array so as to be able torelate acoustic measurements for the latter circumferentially discretesegments of the borehole wall with resistivity measurements of thediscrete portions of the borehole wall opposite individual ones of theelectrodes in the array; detecting acoustic reflections from the wall ofthe borehole; calibrating the acoustic transducers for depths at whichresistivity measurements are made; and deriving from the detectedacoustic reflections and the calibration step an indication of themagnitude of stand-off of individual ones of said electrodes with anaccuracy sufficient to reduce, in said resistivity measurements of earthformations opposite electrodes, an ambiguity attributable to a stand-offcondition.
 25. The method as claimed in claim 24 wherein the calibratingstep includes the steps of:directing acoustic test pulses from a firstcalibrating transducer at a tool located target having a known positionrelative to the first calibrating transducer; detecting acousticreflections from said target as caused by the acoustic test pulses; anddetermining, from said latter detected reflections, calibration signalsrepresentative of the calibration of the stand-off investigatingtransducers as a function of depth.
 26. The method as claimed in claim24 wherein said directing step further comprises directing said pulsesof acoustic energy in the form of beams whose cross-sectional dimensionsare commensurate with the spatial resolution obtained with saidelectrodes.
 27. The method as claimed in claim 24 wherein said directingstep further comprises directing said acoustic pulses from toollocations both above and below said electrodes as viewed along thedirection of investigation of said tool.
 28. The method as claimed inclaim 24 wherein the calibrating step comprises the steps of:directingfirst acoustic test pulses from a first calibrating transducer at afirst target on the tool having a fixed known position relative to thefirst calibrating transducer; detecting reflections of said test pulsesfrom the target; determining, from said detected reflections, signalsrepresentative of the calibration of the formation investigatingtransducer as a function of depth; directing second acoustic test pulsesfrom a fluid measuring transducer through borehole fluid at a secondtarget on the tool having a known distance from said fluid measuringtransducer; detecting reflections from said second target; anddetermining, from said latter reflections, signals representative of theacoustic velocity of the fluid in the borehole as a function of depth.